The water absorbent resin (SAP/Super Absorbent Polymer) is a water-swelling and water-insoluble polymer gelatinizer and is used mainly for disposable purpose, i.e., for absorbing articles such as a disposable diaper and a sanitary napkin and for an agriculture/horticulture water retention agent, an industrial waterproofing material, and the like. Various kinds of monomers and hydrophilic polymers have been proposed as a raw material for such a water absorbent resin. In particular, a polyacrylic acid (salt)-based water absorbent resin in which acrylic acid and/or salt thereof is used as a monomer(s) is industrially most widely used because of its high water absorption performance.
Such a water absorbent resin is produced through a polymerization step, a drying step, an undried substance removing step (if needed), a pulverizing step, a classification step, a surface crosslinking step, and the like (see Patent Literatures 1 through 5).
Moreover, in accordance with advancement of a disposable diaper which is the main purpose of use of the absorbent resin, the water absorbent resin is demanded to have various functions. Specifically, not only a high water absorption capacity, various physical properties such as a gel strength, a water soluble component, a water absorbing speed, a water absorption capacity under load, a liquid permeability, a particle size distribution, a urine resistance, an antibacterial property, an impact resistance (damage resistance), a powder fluidity, a deodorizing property, a fluidity, a filling ability, a coloration resistance (whiteness), and a dust suppression property are requested of the water absorbent resin. Under the circumstances, various proposals concerning surface crosslinking techniques, additives, change in production process, and the like have been made in Patent Literatures below, in addition to the above cited Patent Literatures.
In recent years, among the above described physical properties, the water absorbing speed is becoming a more important factor in accordance with increase in used amount (e.g., 50 wt % or more) of a water absorbent resin in a disposable diaper.
As a method for improving the water absorbing speed, a technique is known by which the water absorbing speed is improved by increasing a specific surface area. Specifically, various techniques have been proposed as follows: that is, a technique to minutely control a particle diameter (Patent Literature 6), a technique to prepare fine particles having a large surface area (Patent Literatures 7 through 9), a technique to causing a hydrogel to become porous by freeze-drying (Patent Literature 10), a technique to carry out surface crosslinking simultaneously with preparing particles (Patent Literatures 11 through 13), a technique of foaming polymerization (Patent Literatures 14 through 32), a technique of foaming and crosslinking after polymerization (Patent Literature 33), and the like.
According to the techniques of Patent Literatures 6 through 33 and the like, the water absorbing speed is increased by, for example, increasing the specific surface area of the water absorbent resin. However, the techniques have a problem such as decrease in bulk specific gravity. The decrease in bulk specific gravity causes problems that, when a container is filled with a water absorbent resin for transportation or the like, (i) a predetermined weight of the water absorbent resin cannot be put in the container or (ii) vibrational operation is required in filling and therefore a water absorbent resin is damaged, and the like. In general, the water absorbing speed and the specific surface area have a positive correlation, and the bulk specific gravity and the specific surface area have a negative correlation. It is therefore difficult to maintain the bulk specific gravity while increasing the water absorbing speed that greatly depends on the specific surface area.
A method for increasing the bulk specific gravity has been proposed before the water absorbing speed is focused on. For example, techniques such as follows have been proposed, i.e., a technique in which a bulk specific gravity and an absorption capacity under load are increased by carrying out surface crosslinking after water absorbent resin particles are polished (Patent Literature 34), and a technique in which a fluidity and a bulk specific gravity are increased by adding a powder lubricant or surfactant (Patent Literature 35). However, in Patent Literature 34, the number of processes for producing a water absorbent resin is increased by the polishing of a surface of the water absorbent resin, and it is also necessary to deal with fine powder generated by the polishing. Therefore, the technique of Patent Literature 34 has a problem that a cost and workload for the production is increased. Moreover, according to a water absorbent resin in Patent Literature 35, the added lubricant and surfactant are eluted into an absorbed liquid and therefore a surface tension of the absorbed liquid is decreased, and this causes increase in returned liquid amount in absorbent articles.
Meanwhile, as a method for improving a fluidity and a transport property which are expected to bring about an effect similar to improvement in bulk specific gravity, techniques such as follows have been proposed, i.e., a technique to add stearic acid and inorganic powder (Patent Literature 36), a technique to add an aggregation controlling agent such as alcohol or polyglycol (Patent Literature 37), and a technique to add metallic soap (Patent Literatures 38 and 39). However, although improvement in bulk specific gravity can be expected by the use of these techniques, not all the techniques necessarily improve the bulk specific gravity. Moreover, according to Patent Literature 37, a water absorbent resin is characterized by being fed well by vibrational filling. However, it may be preferable to fill the container with the water absorbent resin without carrying out vibrational filling, that is, the bulk specific gravity is preferably lower because the number of processes can be reduced and the water absorbent resin would be less damaged. Further, Patent Literature 40 which is an unpublished earlier application also discloses to add a surfactant after surface crosslinking. However, it has been found that, in some of conventional techniques including Patent Literature 40 which is the unpublished earlier application, the surface tension of a water absorbing agent is decreased, and this adversely increases a returned liquid amount in absorbent articles.